1. Field of the Invention
This invention relates to the preparation of catalytic material for use in the selective oxidation of carbon monoxide, to catalyst members comprising such materials and to the conditions of their use. The invention finds utility in the preparation of hydrogen-containing gas streams for use in fuel cells, which generate power by the oxidation of hydrogen.
A known strategy for the use of fuel cells involves the generation of hydrogen from carbonaceous fuels. Generally, this process involves subjecting the fuel to desulfurization, steam reforming and high- and low-temperature water-gas shift reactions. The resulting gas stream comprises significant quantities of hydrogen (H2), carbon dioxide (CO2), water (H2O) and about 0.5% carbon monoxide (CO). The aforesaid quantity of CO is greater than desired for fuel cell purposes, since CO is known to poison the catalyst for the fuel cell reaction. It is therefore necessary to remove some or all of the CO, e.g., by oxidizing it to CO2, without removing the H2 needed to power the fuel cell. The CO must be removed or reduced to a maximum of about 10 ppm. In a prior art process known under the trade name SELECTOXO(trademark), the product of the water-gas shift reactions is stripped of CO in a catalytic selective oxidation process that avoids oxidation of H2. The commercial SELECTOXO(trademark) catalyst involved comprises from 0.3 to 0.5% platinum and 0.03% iron dispersed on alumina support tablets or pellets by wet impregnation of the alumina with a solution of platinum and iron salts. The SELECTOXO(trademark) catalyst material was dried at not more than 125xc2x0 C. because it was expected that that catalyst would be used at temperatures not higher than 125xc2x0 C. and that a higher drying temperature would detrimentally affect the platinum. The catalyzed alumina tablets are typically assembled into a bed through which the feed stream is flowed.
2. Related Art
U.S. Pat. No. 3,088,919 to Brown, Jr. et al, entitled xe2x80x9cTreatment Of Gasesxe2x80x9d and dated May 7, 1963, discloses a process for the preferential oxidation of carbon monoxide in a hydrogen-containing gas, in particular, ammonia synthesis gas. According to the disclosed process, the gas is treated with water and is then passed over a supported platinum catalyst (see col. 1, lines 34-39). The platinum is loaded at from 0.01 to 5 weight percent of the catalyst (col. 3, lines 25-27). The Patent states that the selective oxidation process may be carried out at temperatures from 60xc2x0 F. to 1200xc2x0 F. (15xc2x0 C. to 650xc2x0 C.), preferably 200xc2x0 F. to 450xc2x0 F. (93xc2x0 C. to 232xc2x0 C.) (see col. 3, lines 4-6). Example III discloses a process for oxidizing carbon monoxide using a platinum catalyst at a catalyst temperature bed in the range of 230xc2x0 C. to 500xc2x0 F. (110xc2x0 C. to 260xc2x0 C.). The inlet gas contained 1.7 percent carbon monoxide (CO) and the exit gas contained up to 4,000 parts per million CO. The pressure may be from atmospheric to 300 psig (col. 3, line 7) and the space velocity of the gas through the catalyst may be 100 to 25,000 ft3 gas/ft3 catalyst per hour, preferably 4,000 to 6,000 ft3 gas/ft3 catalyst per hour for a single-stage operation (col. 3, lines 8-17).
U.S. Pat. No. 3,216,783 to Cohn, dated Nov. 9, 1965 and entitled xe2x80x9cProcess For Selectively Removing Carbon Monoxide From Hydrogen-Containing Gasesxe2x80x9d, discloses the use of a supported platinum catalyst containing from 0.01 to 5 weight percent platinum on pelleted, powdered or granulated support material, for use in oxidizing carbon monoxide (col. 1, lines 53-62) at a reaction temperature in the range of 110xc2x0 C. to 200xc2x0 C. (col. 1, lines 35-42). The space velocity of the gas is in the range of from 500 to 100,000 VHSV at 70xc2x0 F. (21.1xc2x0 C.).
U.S. Pat. No. 4,492,769 to Blanchard et al, dated Jan. 8, 1985 and entitled xe2x80x9cPollution Control Catalyst For Internal Combustion Engine Exhaust System/Catalytic Converter and Process For Its Preparationxe2x80x9d, discloses the preparation of certain platinum-containing catalysts with calcining of 300xc2x0 C. (Examples 2, 3, 4, 5).
U.S. Pat. No. 5,583,087 to Slotte, entitled xe2x80x9cMethod For Impregnating Catalyst Support With Platinumxe2x80x9d, dated Dec. 10, 1996, discloses a method for the preparation of a catalytic material that may comprise platinum dispersed on alumina by a wet impregnation technique. In accordance with the teachings of this Patent, a solution of bivalent platinum is prepared, the bivalent platinum is oxidized to Pt+4, e.g., by adding hydrogen peroxide or ozone, and the Pt+4 solution is then impregnated into the support material, preferably via chemisorption. The wetted support material is then calcined at 275xc2x0 C. (see col. 3, lines 29-40).
U.S. Pat. No. 4,818,745 to Kolts, entitled xe2x80x9cCatalyst For Oxidation Of Carbon Monoxide And Process For Preparing The Catalystxe2x80x9d, dated Apr. 4, 1989, discloses a catalyst for the oxidation of carbon monoxide under conditions suitable for laser applications. The catalyst comprises platinum and/or palladium dispersed on alumina via impregnation. The catalyst may contain from 0.5 to 5 weight percent platinum and/or palladium (col. 4, lines 22-29). Iron is also used in the catalyst at a loading of from about 0.2 to 4 weight percent (col. 4, lines 54-57). The preparation method includes drying and calcining the wetted support material in two stages, first at temperatures of about 30xc2x0 C. to about 200xc2x0 C. and then in the range of from about 300xc2x0 C. to about 700xc2x0 C. The material is then subjected to reducing conditions by exposure to a reducing gas at a temperature of about 550xc2x0 C. to 700xc2x0 C. The feed gases for the carbon monoxide oxidation processes described therein are substantially free of hydrogen.
U.S. Pat. No. 4,440,874 to Thompson entitled xe2x80x9cCatalyst Composition And Method For Its Manufacturexe2x80x9d, dated Apr. 3, 1984, discloses a method for the preparation of a catalytic material used for the purification of exhaust gases from internal combustion engines. The catalytic material may comprise platinum and iron and is prepared using a wet impregnation technique to deposit the catalytic metals on an alumina support material. This Patent illustrates drying and calcining the wetted support material at 450xc2x0 C. (col. 7, lines 57-61).
U.S. Pat. No. 4,749,671 to Saito et al entitled xe2x80x9cExhaust Gas Cleaning Catalyst And Process For Production Thereofxe2x80x9d, dated Jun. 7, 1988, discloses a catalytic material useful for cleaning diesel engine exhaust gases or other exhaust gases containing combustible carbonaceous particles. The catalytic material may comprise platinum and iron supported on alumina (see col. 3, lines 40-61). The disclosed method of preparation involves impregnating alumina pellets with a solution of the catalytic metals, drying and calcining the pellets and then grinding them and forming them into a slurry for coating on a carrier. In each example, the wetted support aterial was dried and then calcined at temperatures of 500xc2x0 C. or 600xc2x0 C.
U.S. Pat. No. 4,621,071 to Blanchard et al, dated Nov. 4, 1996, entitled xe2x80x9cComposite Catalyst For Treatment Of Vehicular Exhaust Gases . . . xe2x80x9d, discloses catalytic materials for the treatment of vehicular exhaust gases. The disclosed materials may comprise platinum and iron dispersed on a support material that may be alumina. This Patent teaches that the support material is impregnated with a solution containing the catalytic metals and is then dried and calcined at a temperature of 300xc2x0 C. to 800xc2x0 C. (see col. 6, lines 53-63). The catalyst is then xe2x80x9cactivatedxe2x80x9d by exposure to a reducing atmosphere at a temperature between 200xc2x0 C. and 700xc2x0 C. (see col. 6, lines 64-68). In Examples 3 and 4 of this Patent, impregnated alumina materials were dried at 150xc2x0 C. and then activated at 350xc2x0 C. In Example 6, an impregnated material was dried at 150xc2x0 C. and then calcined at 350xc2x0 C. in air (see col. 11, lines 13-20).
The following references address selective oxidation of carbon monoxide: U.S. Pat. No. 3,631,073 to Cohn, dated December 28, 1971; Canadian Patent 609,619 to Cohn, dated Nov. 29, 1960; Brown, Jr. et al, xe2x80x9cPurifying Hydrogen by Selective Oxidation of Carbon Monoxidexe2x80x9d, 52 Industrial Engineering Chemistry, No. 10, October 1960, page 841; Anderson et al, xe2x80x9cRemoving Carbon Monoxide From Ammonia Synthesis Gasxe2x80x9d, 53 Industrial Engineering Chemistry, No. 8, August 1961, page 645.
U.S. Pat. No. 5,204,302, issued Apr. 20, 1993 to I. V. Gorynin et al, is entitled xe2x80x9cCatalyst Composition and a Method For Its Preparationxe2x80x9d and is hereinbelow referred to as xe2x80x9cthe ""302 Patentxe2x80x9d. The ""302 Patent discloses a multi-layered catalyst material supported on a metal substrate. The metal substrate (column 4, lines 64-68) may be any thermally stable metal including stainless steel and low alloy steel, the ""302 Patent stating that, regardless of which type of substrate is used, there is no appreciable difference in the performance of the bonded layers. As illustrated in FIG. 1 of the Patent and described at column 4, line 32 et seq, a flame spraying or plasma spraying apparatus (FIG. 2 and column 5, line 32 et seq) is used to apply an adhesive sublayer 12 to metal substrate 11, which is shown in solid cross section as a dense (solid) plate-like structure. Adhesive sublayer 12 contains a self-bonding intermetallic compound formed from any one of a number of metal pairings, including aluminum and nickel, as described at column 5, lines 1-6 of the ""302 Patent. The high temperature of the flame or plasma spray operation is said to generate a diffusion layer (13 in FIG. 1) caused by diffuision of material of substrate 11 and sublayer 12 across their interface (column 4, lines 37-41). A catalytically active layer 14 (FIG. 1) is sprayed atop the sublayer 12 and has a gradient composition with an increasing content of catalytically active material as one proceeds away from the interface (column 5, lines 7-24). The catalytically active layer can be alumina, preferably gamma-alumina, and may further include specified metal oxide stabilizers such as CaO, Cr2O3, etc., and metal oxide catalytic materials such as ZrO2, Ce2O3, etc. A porous layer 18 (FIG. 1 and column 5, lines 25-32) contains some catalytically active components and transition metal oxides as decomposition products of pore forming compounds such as MnCO3, Na2CO3, etc., which presumably form pores as gases evolve from the carbonates or hydroxides (column 7, lines 40-45) as they thermally decompose (see column 7, lines 37-45). As described at column 5, line 44 et seq and at column 7, line 37 et seq, sublayer 12, catalytically active layer 14 and porous layer 18 may be applied by a continuous plasma spray operation in which different ones of the powders 21, 28 and 33 (FIG. 2) are fed into the plasma spray in a preselected sequence and at preselected intervals. An optional activator coating 19 may be applied onto the porous layer, preferably by magnetron sputtering (see column 4, lines 56-63 and column 8, lines 24 et seq).
U.S. Pat. No. 4,027,367, issued Jun. 7, 1977 to H. S. Rondeau, which is incorporated herein by reference, is entitled xe2x80x9cSpray Bonding of Nickel Aluminum and Nickel Titanium Alloysxe2x80x9d and is hereinbelow referred to as xe2x80x9cthe ""367 Patentxe2x80x9d. The ""367 Patent discloses a method of electric arc spraying of self-bonding materials, specifically, nickel aluminum alloys or nickel titanium alloys, by feeding metal constituent wires into an electric arc spray gun (column 1, lines 6-13). The ""367 Patent mentions, starting at column 1, line 25, combustion flame spray guns, e.g., guns feeding a mixture of oxygen and acetylene to melt a powder fed into the flame. Such combustion flame spray guns are said to operate at relatively low temperature and are often incapable of spraying materials having melting points exceeding 5,000xc2x0 F. (2,760xc2x0 C.). The ""367 Patent also mentions (starting at column 1, line 32) that plasma arc spray guns are the most expensive type of thermal spray devices and produce much higher temperatures than combustion-type flame spray guns, up to approximately 30,000xc2x0 F. (16,649xc2x0 C.). It is further pointed out in the ""367 Patent that plasma arc spray guns require a source of inert gas for the creation of plasma as well as extremely accurate control of gas flow rate and electric power for proper operation. In contrast, starting at column 1, line 39, electric arc spray guns are stated to simply require a source of electric power and a supply of compressed air or other gas to atomize and propel the melted material in the arc to the substrate or target. The use of electric arc spraying with a wire feed of nickel aluminum or nickel titanium alloys onto suitable substrates, including smooth steel and aluminum substrates is exemplified starting at column 5, line 28, but no mention is made of open, porous or honeycomb-type substrates, or ceramic substrates and there is no suggestion for the use of the resulting articles as carriers for catalytic materials.
U.S. Pat. No. 3,111,396 to Ball, dated Nov. 19, 1963 (hereinafter referred to as xe2x80x9cthe ""396 Patentxe2x80x9d) and entitled xe2x80x9cMETHOD OF MAKING A POROUS MATERIALxe2x80x9d, discloses a method for making a porous metal material or xe2x80x9cmetal foamxe2x80x9d. Essentially, the method comprises forming a porous organic structure such as a mesh, cloth, or a cured foam structure such as an open pore sponge, impregnating the structure with a fluid suspension of powdered metal in a liquid vehicle, and drying and heating the impregnated structure to remove the liquid vehicle and then further heating the organic structure to decompose it and to sinter the metal powder into a continuous form. The resulting metallic structure, while not foamed during the manufacturing process, is nevertheless described as foamed because its ultimate structure resembles that of a foamed material.
SAE (Society of Automotive Engineers) Technical Paper 971032, entitled A New Catalyst Support Structure For Automotive Catalytic Converters by Arun D. Jatkar, was presented at the International Congress and Exposition, Detroit, Mich., Feb. 24-27, 1997. This Paper discloses the use of metal foams as a substrate for automotive catalysts. The Paper describes the use of various metal foams as catalyst substrates and notes that foams made of pure nickel or nickel-chromium alloys were not successful as substrates for automotive catalysts because of corrosion problems encountered in the environment of an automotive exhaust catalyst. Metal foams made from Fecralloy and ALFA-IV(copyright) ferritic stainless steel powders were said to be successful, at least in preliminary tests, for use as substrates for automotive catalysts. A ceramic washcoat having a precious metal loading was deposited onto disks of ALFA-IV(copyright) metal foam produced by Astro Met, Inc. The washcoat comprised gamma-alumina and cerium oxide on which platinum and rhodium in a ratio of 4:1 were dispersed to provide a loading of 40 grams of the precious metal per cubic foot of the foam-supported catalyst. Such catalyzed substrates were said to be effective in treating hydrocarbon emissions.
In an article entitled xe2x80x9cCatalysts Based On Foam Metalsxe2x80x9d, published in Journal of Advanced Materials, 1994, 1(5) 471-476, Pestryakov et al suggest the use of foamed metal as a carrier substrate for catalytic materials for the catalytic neutralization of exhaust gases of car engines. The use of an intermediate layer of high surface area alumina between the metallic foam and the catalytic material is recommended, by direct deposition on the foam carrier. In addition to increasing the surface area of the substrate, the alumina is also credited with protecting the surface of the substrate against corrosion.
SAE Paper 962473 by Reck et al of EMITECH, GmbH, entitled xe2x80x9cMetallic Substrates and Hot Tubes For Catalytic Converters in Passenger Cars, Two- and Three-Wheelersxe2x80x9d, addresses the use of catalytic converters and hot tubes to treat the exhaust of scooters and motorcycles, especially those having two-stroke engines.
A supplier of wire mesh carriers for catalytic materials known as OptiCat offers for sale wire mesh comprising wire that has been plasma spray coated to form a rough surface thereon to improve the adherence of a catalytic material deposited thereon.
Prior art attempts to adhere catalytic materials to metallic substrates include the use of ferrous alloys containing aluminum. The alloy is formed into a substrate structure and is heat-treated under oxidizing conditions. The aluminum oxidizes, forming whiskers of alumina that project from the substrate surface and are believed to provide anchors for catalytic materials. The use of other alloying elements, e.g., hafnium, in ferrous metals for this purpose is known to provide such whiskers upon oxidizing treatment.
In one aspect, the present invention relates to a method of preparing a catalytic material. The method comprises wetting a refractory inorganic oxide support material with a bivalent platinum solution and iron solution and drying and calcining the wetted support material under oxidizing conditions at a temperature in the range from 200xc2x0 C. up to but not including 300xc2x0 C. If the catalytic material is substantially free from at least one of palladium, rhodium and cerium, the temperature range may include 300xc2x0 C.
According to one aspect of the invention, the support material may be a powdered support material such as powdered alumina. Alternatively, the support material may comprise a pelletized support material.
This invention also provides a method of preparing a catalyst member comprising wetting a monolith comprising a refractory material with a bivalent platinum solution and iron solution and drying and calcining the wetted monolith under conditions described above.
The invention also pertains to the catalytic material and catalyst members produced by the methods described herein.
This invention further provides a catalyst member comprising at least one tube mounted in a housing defining two fluid flow paths therethrough, the at least one tube having a catalytic material deposited thereon for exposure to at least one fluid flow path. The catalytic material may be prepared by any one of the methods described above.
This invention further provides a method for oxidizing carbon monoxide in a gas stream containing carbon monoxide, hydrogen and oxygen, comprising contacting the gas stream at a temperature less than 300xc2x0 C. with a catalytic material or catalyst member as described herein.
The method may comprise contacting the gas stream at a temperature of about 90xc2x0 C. with a O2:CO ratio of about 1:1 and a space velocity of about 20,000/hr or, alternatively, at a temperature of about 150xc2x0 C. with a O2:CO ratio of about 1.5:1 and a space velocity of about 80,000/hr.
A further method of this invention for treating a gas containing carbon monoxide, hydrogen and oxygen comprises flowing the gas through a first flow path in a catalyst member having at least two flow paths therethrough with gas in the second flow path whereby to exchange heat between the gases in the two flow paths.